Designers are increasingly using distributed power supply architectures for large electronic equipment. With this type of architecture, electrical power is bussed throughout the equipment at a relatively high dc voltage, such as 48 volts. dc/dc converters mounted near the load (and often on the same printed circuit board as the load) then step this high voltage down to the low voltage required by the load (e.g. 3.3V), typically through an isolating transformer.
These point-of-load dc/dc converters typically have a low height (e.g. 0.5″) so that the designer can place adjacent load boards close together in a card rack. The plan-view size of the converter must also be as small as possible to leave more room on the load board for the load circuitry. Several standard sizes of converters exist, such as the “Full Brick” (2.4″×4.6″), the “Half-Brick” (2.4″×2.3″), and the “Quarter-Brick” (2.4″×1.45″). Other standard and non-standard sizes exist, as well. In general, the larger a dc/dc converter, the more power it can handle.
Typically, dc/dc converters have a flat bottom surface formed by either a housing or potting material. Terminal pins extend from this surface so that the dc/dc converters can be “through-hole mounted” on a printed circuit board (the “PCB”). When the converter's “through-hole pins” are inserted into the PCB's holes, the bottom surface of the converter makes contact with the PCB to ensure its proper positioning in the z-axis direction.
Recently, “open frame” converters have been developed without a housing or potting. To achieve proper z-axis positioning, these converters use plastic or metal “standoffs” that keep the PCB and the converter's substrate separated by a specified distance. Because these standoffs either abut or are attached to the converter's substrate, they take up space on the substrate that could otherwise be used for electronic components. They also partially or totally block the cooling air from flowing under the open frame converter. Finally, the standoff represents an additional cost for the part and for its attachment to the converter.
Most electronic equipment manufactured today uses Surface Mount Technology (SMT) to attach their components to both the top and bottom surfaces of a PCB. In this process, solder paste is first screen-printed onto the PCB in the locations of the component pads. The components are then placed onto the solder paste. Finally, the PCB is passed through a reflow oven in which the solder paste melts and then solidifies during the cool-down stage.
In comparison, dc/dc converters, with their through-hole pins, are attached to the PCB by either manual soldering or by an automated production process called “wave soldering”. With this latter process, the PCB is first preheated and then passed over a molten pool of solder. The solder comes in contact with the bottom of the PCB, and it wicks into the through-holes and solidifies after the PCB leaves the pool of solder.
A typical manufacturing process that requires both SMT and wave soldering would first attach the SMT parts on the PCB, then insert the through-hole components, and finally pass the PCB through the wave soldering machine. This process requires that the SMT components mounted on the bottom side of the PCB pass through a molten pool of solder.
As the distance between the leads on SMT packages gets smaller, it becomes more difficult to pass these packages through a wave solder process and not have solder bridges form between adjacent leads. Furthermore, the heating associated with the wave soldering process compromises the quality of the SMT components and their attachments to the PCB. Manufacturers of electronic equipment are therefore interested in avoiding the use of wave soldering altogether. Often, the dc/dc converter is the only component on their boards that requires wave soldering.
In response to this desire, several power supply manufacturers have created dc/dc converters designed to be surface mounted to a PCB. Instead of a few, large diameter through-hole pins, some of these converters have many smaller leads designed for surface mounting. In general, these surface mount pins make a dc/dc converter's overall footprint larger than it might otherwise be since the pins typically extend beyond the converter's original footprint. Alternatively, at least one manufacturer has introduced a product that uses a surface mountable ball-grid. In this product, each through-hole pin of a standard converter is replaced with a conductive ball of sufficient diameter to permit the converter to be attached to the PCB with SMT techniques.
One important problem with all of these approaches for making a surface mountable dc/dc converter is the relative weakness of a surface mount joint compared to a through-hole pin. This problem is particularly important since dc/dc converters have a higher mass than most components, and the mounting joints are therefore more susceptible to shock and vibration stresses.
Another problem with a surface mountable dc/dc converter is that the converter's pins make electrical contact with only the outer conductive layer in the PCB. Normally, the PCB's power and ground planes use inner conductive layers. With a surface mount connection, additional vias (that take up space and add resistance) are therefore required to connect the outer conductive layer to the inner ones.
In comparison, a through-hole mounting is much stronger mechanically. It also provides direct electrical attachment of the pin to the inner conductor layers of the PCB.
What is needed is a way to solder a through-hole mounted pin with a reflow solder process, instead of using manual or wave soldering.